1. Field of the Invention
The present invention relates to a light stabilizer emulsion composition for imparting weatherability to various aqueous coating agent compositions and to an aqueous coating agent composition containing the light stabilizer emulsion composition.
2. Description of the Related Art
The number of coating agent compositions not using organic solvents but water has been increasing in consideration of, for example, recent environmental problems. Examples of applications of such coating agent compositions include coating applications such as aqueous paint, aqueous ink, optical coating agents, agents for fibers, and agents for paper. A coating agent composition to be used for such applications is added with a light stabilizer to improve the weatherability of the formed coating film in some cases. The term “light stabilizer” as used herein refers to, for example, a hindered amine compound for capturing radicals generated by deterioration or a UV absorber for capturing ultraviolet.
However, such light stabilizers have a problem in that, when the light stabilizer is added to an aqueous composition, the blended light stabilizer is not dispersed well, so a formed coating film does not have sufficient weatherability because the light stabilizer is generally an oil-soluble organic compound. Light stabilizers have another problem in that light stabilizers added to coating agent compositions separate during the storage of the composition, thereby resulting in poor storage stability.
To cope with the foregoing, in, for example, an application where an aqueous emulsion is used, the addition of a light stabilizer in a step of polymerizing the aqueous emulsion has been known. For example, JP 11-080486 A discloses an acryl silicone emulsion composition obtained by the emulsion polymerization of a mixture containing: a monomer mixture composed of 30 to 96 wt % (mass %) of a (meth)acrylate monomer (1), 1 to 10 wt % (mass %) of one kind of specific polymerizable carboxylic acid (2) or a mixture of two or more kinds of such carboxylic acids, 1 to 10 wt % (mass %) of a polymerizable emulsifier (3), 1 to 20 wt % (mass %) of a benzophenone- or benzotriazole-based polymerizable light stabilizer (4), and 1 to 30 wt % (mass %) of an organoalkoxysilane compound having at least one glycidyl group in one molecule thereof (5) (the total amount of them is 100 wt % (mass %)); and 0.1 to 5 wt % (mass %) of a hindered amine-based light stabilizer (HALS) having a base constant (PKb) of 8 or more (6) (claim 1).
In addition, JP 2002-285045 A discloses an aqueous paint composition containing (A) a fluoroolefin-based copolymer emulsion and (B) a copolymer emulsion containing a UV absorber and/or a light stabilizer obtained by the emulsion polymerization of a mixture of: a monomer mixture composed of (a) 5 to 70 wt % (mass %) of a polymerizable unsaturated monomer having a cycloalkyl group and (b) 30 to 95 wt % (mass %) of any other polymerizable unsaturated monomer; and (c) the UV absorber and/or the light stabilizer (claim 1). In addition, the paragraph [0016] of JP2002-285045A discloses a UV absorber such as a salicylic acid derivative or a benzophenone- or benzotriazole-based UV absorber and a light stabilizer such as a hindered amine derivative.
A method involving: emulsifying or dispersing a light stabilizer into water in advance; and adding the resultant to an aqueous emulsion has been also known. For example, JP 2003-261759 A discloses: a light stabilizer aqueous emulsion containing a light stabilizer, an emulsifier, and water, in which the content of the light stabilizer is 20 to 85 mass % (claim 1); and the light stabilizer aqueous emulsion in which the light stabilizer is a triazine-based UV absorber (claim 3). The light stabilizer aqueous emulsion can be added to an aqueous emulsion or an aqueous coating agent composition later. Therefore, the light stabilizer aqueous emulsion has an advantage in that it has an extremely simple structure and easily exhibits the function of so-called weatherability.
According to JP 11-080486 A and JP 2002-285045 A described above, a light stabilizer can be uniformly dispersed into an aqueous emulsion. However, the light stabilizer is captured by emulsion particles, so there arises a problem in that the formed coating film does not have sufficient weatherability.
In addition, the light stabilizer aqueous emulsion of JP 2003-261759 A prepared by emulsifying or dispersing a light stabilizer into water can be added to an aqueous emulsion or an aqueous coating agent composition later. Therefore, the light stabilizer aqueous emulsion has an advantage in that it has an extremely simple structure and easily exhibits the function of so-called weatherability. However, such prior art has a problem in that separation or precipitation is apt to occur, so storage stability is poor. Furthermore, the formation of a coating film by means of an aqueous coating agent composition to which such a compound of the prior art has been added involves the emergence of reductions in various physical properties of the coating film such as the deterioration of gloss and insufficient color development.